The invention relates to a method of drilling a borehole into subsurface earth formations using a rotary drill bit.
In conventional drilling operations the drill bit is usually coupled to the lower end of a single-bore drill string. During drilling a drilling mud is circulated down through the drill string and up through the pipe/formation annulus between the pipe string and the borehole wall. The circulated drilling mud has three basic functions: to cool the bit, to carry cuttings to the surface and to keep the wellbore under control.
A drilling mud with sufficient viscosity, fluid-loss control and density to fulfill these functions is inherently a poor fluid for achieving a high penetrations rate of the bit. In practice, compromise fluid formulations are used and penetration rate is usually the parameter which has to compromise the most.
The invention aims to provide a method of drilling a borehole which enables a high drilling penetration rate to be achieved without making concessions to the degree of control of the wellbore.